While a number of principles underlying cell fate specification have been discovered, the mechanisms that create such vast neuronal diversity are still not well understood. Neural differentiation is often thought of as a two step process, one that establishes general neural fate and a second that further differentiates cell types. This specification can be taken a step further as in bilateral organisms, initially similarly-fated neurons on the right and left sides often acquire different functions. This proposal will study links between these steps of differentiation by analyzing a gene that affects the acquisition of both general and specific cell fates in the Caenorhabditis elegans ASE taste neurons. In a screen for lateral asymmetry defects in the bilaterally symmetric ASE neurons, one surprising mutant affects the neurons' ability to not only acquire left and right fate, but also to acquire ASE fate altogether. It is the first mutant of its kind to be pulled out of a screen in this context. Within this proposal, the mutant will be characterized, mapped and cloned, and functionally dissected. Initial observations suggest that this gene could be involved both in establishing ASE cell fate and in promoting the asymmetric expression of a number of right and left cell fate markers. [unreadable] [unreadable]